Arenaviruses are important human pathogens with several of them causing hemorrhagic fever (HF) disease that pose an important public health problem in their endemic regions. In addition, the worldwide-distributed prototypic arenavirus LCMV is a neglected human pathogen of clinical significance. Moreover, arenaviruses represent a credible biodefense threat. There are not FDA-licensed vaccines and current anti-arenavirus therapy is limited to the use of ribavirin, which is only partially effective. Therefore, there is n unmet need for the development of novel anti-arenaviral therapeutics, which will be facilitated by the characterization of arenavirus-host cell protein interactions contributing to virus multiplication. Activity-based protein profiling (ABPP) is a novel approach that permits to monitor the effects of viral infections on the functional state of the host proteome to identify novel viru-host protein interactions that may affect cell physiology, virus propagation and pathogenesis. The goal of this exploratory R21 application is to use ABPP of serine hydrolases (SHs) to identify novel arenavirus-host interactions, as well as targets and candidate drugs, to develop strategies to combat arenavirus infections. We focus on SHs because: 1) SHs represent 1% of the mammalian proteome and are involved in many physiological and pathological processes. 2) We have observed altered SH activities in LCMV-infected cells and some SH inhibitors exhibited anti-LCMV activity. Our specific aims (SA) are: SA1. Identify SH inhibitors with anti-arenaviral activity. We will test the hypothesis that specific SH contribute to the arenavirus life cycle and that their inhibition will interfere with virus multiplication. We will use a novel cell-based assay to screen a library of 250 SH inhibitors to identify those with anti-LCMV activity. Candidates with high inhibitory potency (EC90 ? 5 M) and therapeutic index (TI = CC50/EC90) ? 30 will be selected for studies aimed at defining molecular targets and mechanism of action. SA2. Validation and characterization of selected hits. We will test the hypothesis that SH inhibitors with anti- LCMV activity inhibit specific steps of the virus life cycle. We will use established cell-based assays to determine the effects of selected hits on cell entry, RNA synthesis and budding of LCMV, as well as Lassa and Junin the two HF arenaviruses with the highest impact in human health. SA3. Identify targets of SH inhibitors with anti-arenaviral activity. We will test the hypothesis that SH inhibitors with anti-arenavirus activity interact wit and inhibit specific host SH. Selected hits will be functionalized with a photo-reactive diazirine group to covalently modify interacting protein targets and a bio-orthogonal alkyne handle to enable click chemistry, to biotin to facilitate enrichment and subsequent mass spectrometry analysis using multi-dimensional protein identification technology (MudPIT). We will use complementary genetic and pharmacological approaches to validate the contribution of identified targets to the arenavirus life cycle.